Classification of path attributes

There are four categories of path attributes:

Well-known mandatory

Must be recognized by all BGP routers and must be included in every update message. Routing information errors occur without this attribute.

Well-known discretionary

Can be recognized by all BGP routers; can be included in every update message as needed.

Optional transitive

Transitive attribute between ASs. A BGP router not supporting this attribute can still receive routes with this attribute and advertise them to other peers.

Optional non-transitive

If a BGP router does not support this attribute, it will not advertise routes with this attribute.

The category of each BGP path attribute is described in the following table.